Solid electrolyte detectors for the on-line monitoring of sulfur-bearing pollutants SO.sub.2, SO.sub.3, i.e. SO.sub.x, have recently been developed and are described in detail in the issued Canadian Pat. No. 1,040,264, entitled "Solid State Sensor For Anhydrides", issued Oct. 10, 1978, which is assigned to the assignee of the present invention and incorporated herein by reference. This inventive concept is the subject matter of pending U.S. Patent application Ser. No. 718,511 now U.S. Pat. No. 4,282,078. The operation of the SO.sub.x detector described in the above-referenced patent and patent application is based on potentiometric measurements across a solid electrolyte element of potassium sulfate (K.sub.2 SO.sub.4), wherein accurate measurements of sulfur-bearing pollutants over a concentration range of 0.1 parts per million to 10,000 parts per million can be realized. The sensor thus described is uniquely sensitive to SO.sub.x. The presence of other common pollutants such as CO.sub.2, CH.sub.4 and NO.sub.x does not interfere with the SO.sub.x measurements.